Organic EL devices have received a great deal of attention because they can be used to produce displays and light sources that have the advantage of being flexible, thin, and lightweight. However, organic EL devices still have many problems, and further improvement in their characteristics is desired in order to expand their use.
As one of techniques for improving the characteristics of organic EL devices, a technique of interposing a hole-injection layer at the interface between an anode and a hole-transport layer has been known. In addition, it has been reported that the characteristics can be improved by using a MoO3 layer as the hole-injection layer. Such information is disclosed in JP 2005-32618 A and JP 2006-344774 A. Moreover, S. Tokito et al. also reported such information (J. Phys. D: Appl. Phys., 29, 2750 (1996)). Moreover, T. Miyashita et al. also reported such information (Jpn. J. Appl. Phys., 44, 3682 (2005)). Moreover, C.-W. Chen et al. also reported such information (Appl. Phys. Lett., 87, 241121 (2005)). Moreover, R. Satoh et al. also reported such information (Jpn. J. Appl. Phys., 45, 1829 (2006)). Moreover, H. You et al. also reported such information (J. Appl. Phys., 101, 026105 (2007)). The thicknesses of MoO3 layers that generally have been used were in a range of 2 to 50 nm.
The object of the invention disclosed in JP 2006-344774 A is to increase the brightness and to reduce the power consumption of an organic EL device. JP 2006-344774 A discloses a top emission type organic EL device that emits light from the side opposite to a substrate. In this organic EL device, an anode formed of aluminum, an organic layer, and a cathode formed of ITO are stacked on the substrate in this order. The light generated in the organic layer is emitted outward through the cathode. A Mo oxide layer (thickness is, for example, 3.5 to 1000 angstroms) is disposed between the anode and the organic layer. In the case where the anode of the top emission type organic EL device mentioned above includes a transparent electrode such as ITO, the light heading for the anode from the light emitting layer passes through the transparent electrode twice before exiting the device. Because of this, JP 2006-344774 A teaches that when the top emission type organic EL device mentioned above includes a transparent electrode such as ITO, the optical absorption of the transparent electrode becomes a problem (paragraph [0004] of JP 2006-344774 A). Moreover, JP 2006-344774 A teaches that an anode formed of ITO causes a problem in that the current density decreases (paragraph [0005] of JP 2006-344774 A).